Light controller and imaging apparatus

ABSTRACT

The invention relates to a light control device and an imaging device those are enabled to realize the enhancement of optical functions thereof.  
     The light control device is provided at a front position of an optical lowpass filter  55   b  in an optical path of an imaging system of an imaging device, and comprises a liquid crystal element and a polarizing plate. Further, the light control device is configured so that the direction of a polarization axis  14  of the polarizing plate and the direction of liquid crystal alignment of the liquid crystal element differ from light ray separation directions of an ordinary ray and an extraordinary ray, which are separated by an optical lowpass filter  55   b.  The light control device can be applied to the optical diaphragm of the imaging device, such as the CCD camera, MOS image sensor and the like.

TECHNICAL FIELD

The present invention relates to a light control device, for example,for controlling a quantity of incident light and for outputting thelight, and to an imaging device using this light control device.

BACKGROUND ART

Generally, a polarizing plate is used in a light control device using aliquid crystal cell. As this liquid crystal cell, for instance, a TN(Twisted Nematic) liquid crystal cell or a Guest Host (GH) liquidcrystal cell is used.

FIGS. 12A and 12B are schematic drawings each showing the principle ofan operation of a conventional light control device. This light controldevice is constituted mainly by a polarizing plate 1 and a GH cell 2.The GH cell 2, which is not shown in the drawings, is sealed between twoglass substrates, and has a working electrode and a liquid crystalalignment film (this also applies to the following cases). A positiveliquid crystal molecule 3 and a positive dichroic dye molecule 4 aresealed in the GH cell 2.

The positive dichroic dye molecule 4 has anisotropy of light absorption,and is, for example, a positive (p-type) dye molecule. Further, thepositive liquid crystal molecule 3 is of the positive type (plus type),whose anisotropy of dielectric constant is positive.

FIG. 12A shows a state of the GH cell 2 during a voltage is not appliedthereto (no voltage is applied thereto). Incident light 5 is transmittedby the polarizing plate 1 thereby to be linearly polarized. In FIG. 12A,this polarization direction coincides with a molecular long axisdirection of the positive dichroic dye molecule 4. Thus, light isabsorbed by the positive dichroic dye molecule 4, so that the lighttransmittance of the GH cell 2 is reduced.

Further, as shown in FIG. 12B, a voltage is applied to the GH cell 2. Asthe positive liquid crystal molecule 3 is directed toward the directionof an electrical field, the molecular long axis of the positive dichroicdye molecule 4 is perpendicular to the polarization direction of thelinearly polarized light. Therefore, the incident light 5 is transmittedalmost without being absorbed by the GH cell 2.

Incidentally, in the case of using a negative type (n-type) dichroic dyemolecule, which absorbs light in a molecular short axis direction,conversely to the case of using the positive dichroic dye molecule 4,when no voltage is applied thereto, light is not absorbed, whereas lightis absorbed when a voltage is applied thereto.

In the light control device shown in FIGS. 12A and 12B, the ratio ofabsorbance on application of a voltage to absorbance on application ofno voltage, that is, an optical density ratio is about 10. This devicehas an optical density ratio that is about twice the optical densityratio of the light control apparatus constituted only by the GH cell 2without using the polarizing plate 1.

On the other hand, ordinary video cameras and digital still cameras eachhave CCDs (Charge Coupled Devices) for converting the intensity of lightinto electrical signals. A single CCD has several hundred thousand toseveral million pixels.

Further, a color filter is provided corresponding to each of the pixels.For instance, in a case where a striped pattern or the like having awidth, which is equal to that of this colored CCD pixel, is imaged, apart of color signals to be formed originally of three colors, that is,red, green, and blue is lacked. Thus, a color differing from an originalcolor comes out. Also, a non-colored part is colored. Consequently, animage of the pattern or the like is very hard to see under suchinfluences of false signals.

That is, the CCDs perform geometrically discrete sampling. This causestroubles that false color signals and moirés occur when geometricalpatterns (of, for example, striped clothes, and tiled walls ofbuildings) finer than the periodic arrangement of the CCDs are shown,and that images of the patterns causes feeling of incongruity.

As a countermeasure thereagainst, recently, there has generally beenemployed means for preventing generation of false color signals byinstalling an optical lowpass filter, which is constituted by abirefringent plate made of quartz or the like, at a front position ofthe CCD thereby to blur high-frequency components of a striped patternand so on and to make the striped pattern not to look like stripes andalso make it clear which of a striped pattern or a color is shown.

Referring to FIGS. 14A and 14B, which illustrate a concrete principle,when natural light 31 having random oscillating directions is incidentupon a birefringent plate 32 made of quartz or the like, the naturallight 31 is split into an ordinary ray 33 and an extraordinary ray 34.Thus, a light ray forming an image at a single point is split to thoserespectively forming images at two points. A splitting axis d can becalculated according to the following equation (1). For instance, thesplitting axis d is expressed as being about 5.9×10⁻³×t. $\begin{matrix}{d = {\frac{( n_{e} )^{2} - ( n_{o} )^{2}}{2{n_{e} \cdot n_{o}}} \times t}} & {{equation}\quad(1)}\end{matrix}$(incidentally, in the equation (1), t designates a thickness of thebirefringent plate, and n_(o) denotes the refractive index of theordinary ray, and n_(e) designates the refractive index of theextraordinary ray).

For example, in a case where two birefringent plates differing incrystal axis from each other are combined with each other, as shown inFIG. 15A, what is called rhombic four-point blurring can be performed.In a case where three birefringent plates differing in crystal axis fromone another, are combined with one another, as shown in FIG. 15B, whatis called seven-point blurring can be performed. Also, in a case of acombination of three plates, in which a phase difference plate issandwiched by birefringent plates, as shown in FIG. 15C, what is calledsquare four-point blurring can be performed. Incidentally, becausepixels are usually formed in a square arrangement in a digital stillcamera, the square four-point blurring shown in FIG. 15 has generallybeen employed. However, recently, with miniaturization of CCDs,employment of devices of the type having two birefringent plates made ofquartz has been increased, in view of the balance between the necessityfor enhancing the frequency characteristics and the cost thereof.

Additionally, regarding the digital cam coders, nearly similar opticallowpass filters have been employed.

However, it turns out that even in a case where the optical lowpassfilter is disposed at a front position of the CCD, as shown in FIGS. 12Aand 12B, and where an object, whose spatial separation capability ishigh, is imaged by using an imaging device, on which a light controldevice constituted by the GH cell 2 and the polarizing plate 1, as shownin FIGS. 12A and 12B, is mounted, a mode constituted by the GH cell 2and the polarizing plate 1, the action of separation between an ordinaryray and an extraordinary ray due to birefringerence does not effectivelyfunction owing to the positional relation between the device and theoptical axis of each of the birefringent plates, that a deviation of theintensity of the separated ray occurs, and that the effect of preventingthe generation of a false color signal is insufficient. Improvementthereon has been desired.

The invention is accomplished to the above-mentioned problems. An objectof the invention is to provide a light control device enabled to realizethe enhancement of optical functions thereof, and to provide an imagingdevice enabled to realize the enhancement of the performance, thequality of picture, and the reliability thereof by disposing this lightcontrol device in an optical path thereof.

DISCLOSURE OF THE INVENTION

That is, according to the invention, there is provided a light controldevice provided at a front position of an optical lowpass filter in anoptical path of an imaging system of an imaging device. The lightcontrol device comprises a liquid crystal element and a polarizingplate. The direction of a polarization axis of the polarizing plate andthat of liquid crystal alignment of the liquid crystal element differfrom light ray separation direction of an ordinary ray and anextraordinary ray, which are separated by an optical lowpass filter.

Also, according to the invention, there is provided an imaging device inwhich a light control device having a polarizing plate and a liquidcrystal element is disposed at a front position of an optical lowpassfilter in an optical path of an imaging system. The imaging device isconfigured so that the direction of a polarization axis of thepolarizing plate and the direction of a liquid crystal orientationdiffer from light ray separation direction of an ordinary ray and anextraordinary ray separated by the optical lowpass filter.

Incidentally, the “direction of the liquid crystal orientation” means adirection in which liquid crystal molecules are arranged on a substratesurface of a liquid crystal element, that is, a direction (for example,a rubbing direction) in which liquid crystal molecules are aligned whenprojected on a surface perpendicular to the optical path.

According to the invention, the device is configured so that thedirection of a polarization axis of the polarizing plate and thedirection of a liquid crystal orientation differ from light rayseparation direction of an ordinary ray and an extraordinary rayseparated by the optical lowpass filter. Thus, for instance, whengeometrical patterns (of, for example, objects, which have high spatialfrequencies, to be imaged, such as striped clothes, and tiled walls ofbuildings), which are finer than the periodic arrangement of the CCDs,are imaged, no deviation of the intensity of the separated light occurs.The effect of blurring can sufficiently be obtained. Occurrences offalse color signals and moirés can effectively be prevented. A picked-upimage, which has faithfully reproduced an imaged object and is a morenatural image, can be obtained. Even when the directions of thepolarization axis and the liquid crystal orientation are parallel to thelight ray separation direction, a deviation of the intensity of theseparated light occurs, so that the number of the separated light raysdecreases, and that the effect of blurring is not obtained.

Therefore, the invention can enhance the optical functions of the lightcontrol device and the imaging device and is extremely effective inenhancing the performance, the quality of an image, and the reliabilityof the device.

BRIEF DESCRIPTION of DRAWINGS

FIG. 1 is a view showing the direction of a polarization axis of apolarizing plate and an example of the configuration of an opticallowpass filter according to a mode for carrying out the invention;

FIG. 2 is a schematic side view illustrating a light control deviceusing a liquid crystal element according to the mode for carrying outthe invention;

FIG. 3 is a front view illustrating a mechanical iris of the lightcontrol device according to the mode for carrying out the invention;

FIGS. 4A, 4B, and 4C are schematically and partially enlarged viewsillustrating an operation of the mechanical iris provided in thevicinity of an effective optical path of the light control deviceaccording to the mode for carrying out the invention;

FIG. 5 is a view illustrating the process of rubbing a liquid crystalalignment film in manufacturing the liquid crystal element according tothe mode for carrying out the invention;

FIG. 6 is a view illustrating an example of the combination of thedirections of the polarization axis of the polarizing plate and theliquid crystal orientation of the liquid crystal element according tothe mode for carrying out the invention;

FIG. 7 is a view illustrating an example of the combination of thedirections of the polarization axis of the polarizing plate and thedirection of the liquid crystal alignment of the liquid crystal elementaccording to the mode for carrying out the invention;

FIG. 8 is a schematic cross-sectional view illustrating a camera systemincorporating the light control device according to the mode forcarrying out the invention;

FIGS. 9A and 9B are views illustrating an algorithm for controlling thelight transmittance in the camera system according to the mode forcarrying out the invention;

FIGS. 10A, 10B, and 10C are schematic views illustrating the principleof an operation of the light control device according to an inventiondescribed in an earlier application (Japanese Patent Application No.11-322186 Official Gazette);

FIG. 11 is a graph illustrating the relation between the lighttransmittance of a light control device according the inventiondescribed in the earlier application and a drive applied-voltage;

FIGS. 12A and 12B are schematic views illustrating the principle of anoperation of a conventional light control device;

FIG. 13 is a graph illustrating the relation between the lighttransmittance of the conventional light control device and a driveapplied-voltage;

FIGS. 14A and 14B are views illustrating an action of a birefringentplate constituting an optical lowpass filter; and

FIGS. 15A, 15B, and 15C are views illustrating an action of thebirefringent plate constituting the optical lowpass filter.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventor zealously studied improvement of theabove-mentioned problems of the generation of false color signals andmoirés, and, for the first time, knew that in some manners of mountingthe liquid crystal element and the polarizing plate, which constitutethe light control device, the deviation of the intensity of theseparated light occurs and the optical lowpass filter's effect ofpreventing the generation of false color signals is reduced. The presentinventor has attained the invention by ascertaining that it is extremelyeffective for the improvement thereof to place each of the polarizationaxis of the polarizing plate, the liquid crystal orientation of theliquid crystal element, and the optical axis of the optical lowpassfilter in an optimal direction.

That is, it is important for the invention to configure the device sothat the direction of the polarization axis of the polarizing plate andthe direction of the liquid crystal alignment differ from the light rayseparation direction of the ordinary ray and the extraordinary rayseparated by the optical lowpass filter.

Incidentally, it is preferable that the direction of the polarizationaxis of the polarizing plate and the direction of liquid crystalalignment of the liquid crystal element differ from at least the lightray separation direction of a birefringent plate, which is presentclosest to a light incidence side among members constituting the opticallowpass filter and which is made of quartz or the like.

More concretely, the direction of the polarization axis of saidpolarizing plate and that of liquid crystal alignment of said liquidcrystal element forms an angle of 45 degrees with at least the light rayseparation direction of a birefringent plate that is present closest toa light incidence side among members constituting the optical lowpassfilter. Consequently, a light ray, which forms an image at one point, iseasily split into those corresponding to square four points, as shown inFIG. 15C. Thus, the device according to the invention can be made to bemore suitable for, for example, miniaturization of a CCD (Charge CoupledDevice).

Also, it is preferable that the polarization axis of the polarizingplate and the direction of the liquid crystal orientation of the liquidcrystal element are perpendicular to each other. Consequently, the ratioof absorbance on application of a voltage to absorbance on applicationof no voltage (that is, an optical density ratio) is enhanced. Thecontrast ratio of the light control device is increased. A light controloperation can normally be performed at all places from light places todark places.

According to the light control device and the imaging device based onthe present invention, the direction of the polarization axis of thepolarizing plate and that of the liquid crystal orientation of theliquid crystal element are set to differ from, for instance, form anangle of 45 degrees with at least the light ray separation direction ofthe birefringent plate that is present closest to the light incidenceside among the members constituting the optical lowpass filter. Thus,for example, even in a case where the light control device, the opticallowpass filter, and a CCD element according to the invention aredisposed in a casing, and where even when geometrical patterns (of, forexample, objects, which have high spatial frequencies, to be imaged,such as striped clothes, and tiled walls of buildings), which are finerthan the periodic arrangement of the CCDs, are imaged, no deviation ofthe intensity of the separated light occurs. The effect of blurring cansufficiently be obtained. The generation of false color signals andmoirés can effectively be prevented. A picked-up image, which hasfaithfully reproduced an imaged object and is a more natural image, canbe obtained.

Furthermore, it is preferable that the liquid crystal element is aguest-host liquid crystal element that employs negative liquid crystalmolecules as a host material, and that also employs dichroic dyemolecules as a guest material.

Such a liquid crystal element is based on the invention described in theearlier application, which the present inventor already filed, accordingto the Japanese Patent Application No. 11-322186 Official Gazette. Inaccordance with the invention described of the earlier application, thelight control device is constituted by the liquid crystal element andthe polarizing plate disposed in the optical path of light that isincident upon this liquid crystal element. Moreover, the ratio ofabsorbance on application of a voltage to absorbance on application ofno voltage (that is, the optical density ratio) is enhanced by using theguest-host liquid crystal that employs negative liquid crystal moleculesas the host material. The contrast ratio of the light control device isincreased. This enables the device to normally perform a light controloperation at all places from light places to dark places.

In the guest-host liquid crystal cell (the GH cell) 2 shown in FIGS. 12Aand 12B, positive liquid crystal molecules, whose dielectric constantanisotropy (Δε) is positive, are used as the host material. Positivedichroic dye molecules 4, whose light absorption anisotropy (ΔA) ispositive, are used as the guest material. The polarizing plate 1 isdisposed at the light incidence side of the GH cell 2. Change in thelight transmittance on the application of an operating voltage ismeasured by using rectangular waves as drive waves. Thus, as shown inFIG. 13, as the operating voltage is applied, an average lighttransmittance of visible light (in the air, the transmittance measuredby adding the polarizing plate to the liquid crystal is used as areference (100%): this also applies to the following cases) increases.However, when the voltage is raised to 10 V, a maximum lighttransmittance is 60% or so. Additionally, the change in the lighttransmittance is gentle.

This is considered to be because of the facts that the interaction ofthe liquid crystal molecules in the interface between the liquid crystalalignment film of the liquid crystal cell and each of the liquid crystalmolecules is strong on the application of no voltage in the case ofusing the positive host material, and that thus, even when a voltage isapplied thereto, the liquid crystal molecules, whose director does notchange (or is difficult to change) orientation thereof, still remain.

In contrast, according to the invention described in the earlierapplication, as shown in FIGS. 10A to 10C, in the guest-host liquidcrystal cell (the GH cell) 12, MLC-6608 manufactured by Merck Ltd.,which is a negative liquid crystal whose dielectric constant anisotropy(Δε) is positive, is used as the host material. As the guest material,D5, which is positive dichroic dye molecules and manufactured by BDHLtd., is used by way of example. Thus, the polarizing plate 11 isdisposed at the light incidence side of the GH cell 12. Change in thelight transmittance on application of the operating voltage was measuredby using square waves as drive waves. Thus, it turns out that as theoperating voltage is applied, the average light transmittance of visiblelight (in the air) reduces to several % from the maximum lighttransmittance of about 75%, as shown in FIG. 11, and that the change inthe light transmittance is relatively rapid.

This is considered to be because of the facts that the interaction ofthe liquid crystal molecules in the interface between the liquid crystalalignment film of the liquid crystal cell and each of the liquid crystalmolecules is extremely weak on the application of no voltage in the caseof using the negative host material, and that thus, when no voltage isapplied, light is easily transmitted, whereas when a voltage is appliedthereto, the orientation of the director of the liquid crystal moleculeis liable to change.

Thus, according to the invention, the GH cell 12 is constituted by usingthe negative host material thereby to enhance the light transmittance(especially, when the cell is transparent) and to realize a more compactlight control device enabled to use the GH cell 12 by fixing theposition thereof in the imaging optical system without change. In thiscase, the polarizing plate is disposed in the optical path of light thatis incident upon the liquid crystal element. Consequently, the ratio ofabsorbance on application of a voltage to absorbance on application ofno voltage (that is, the optical density ratio) is further enhanced. Thecontrast ratio of the light control device is further increased. A lightcontrol operation can normally be performed at all places from lightplaces to dark places.

Incidentally, in the device of the invention, preferably, the liquidcrystal element is negative liquid crystal molecules whose dielectricconstant anisotropy is negative. However, the guest material may beconstituted by positive or negative dichroic dye molecules. Further,although it is preferable that the host material is negative liquidcrystal molecules, positive liquid crystal molecules may be used as thehost material.

In the device of the invention, the negative (or positive) host materialand the positive (or negative) guest material can be selected from knownmaterials. Incidentally, in the case of practically using the device,the device may use a composition obtained by selecting and blending thematerials in such a way as to show a nematic property in a practicaloperating temperature range.

Further, it is preferable that the polarizing plate 11 constituting thislight control device is enabled to be taken in and out of an effectiveoptical path of light, which is incident upon the GH cell 12, as shownin FIG. 2, similarly to the polarizing plate according to the inventiondescribed in the earlier application that is filed by the applicant ofthe present application. Concretely, the polarizing plate 11 can betaken out of the effective optical path by being moved to a positionindicated by imaginary lines. A mechanical iris shown in FIG. 3 may beused as the means for taking this polarizing plate 11 therein and outtherefrom.

This mechanical iris is a mechanical iris diaphragm generally used in adigital still camera and a video camera, and the like, and mainlycomprises two iris blades 18, 19, and the polarizing plate 11 attachedto the iris blade 18. The iris blades 18 and 19 can be moved in anup-down direction. In a direction indicated by an arrow 21, the relativemovement of the iris blades 18 and 19 is performed by using the drivemotor (not shown).

Consequently, as shown in FIG. 3, the iris blades 18 and 19 arepartially overlapped with each other. When these blades largely overlapwith each other, an opening 22 positioned near the center between theiris blades 18 and 19 on the effective optical path 20 is covered withthe polarizing plate 11.

FIGS. 4A to 4C are partially enlarged views illustrating the mechanicaliris provided in the vicinity of the effective optical path 20.Simultaneously with the downward movement of the iris blade 18, the irisblade 19 upwardly moves. Along with this, the polarizing plate 11attached to the iris blade 18 is moved out of the effective optical path20, as illustrated in FIG. 4A. Conversely, the iris blades 18 and 19 areoverlapped with each other by moving the iris blade 18 upwardly, andalso moving the iris blade 19 downwardly. Consequently, as shown in FIG.4B, the polarizing plate 11 is moved onto the effective optical path 20and gradually covers the opening 22. When the iris blades 18 and 19largely overlap with each other, the polarizing plate 11 covers theentire opening 22, as shown in FIG. 4C.

Next, a light control operation of the light control device using thismechanical iris is described.

As an object (not shown) to be imaged becomes brighter, the iris blades18 and 19 having been opened in the up-down direction are driven by amotor (not shown), and start overlapping with each other, as shown inFIG. 4A. Thus, the polarizing plate 11 attached to the iris blade 18starts going onto the effective optical path 20, and covers a part ofthe opening 22 (FIG. 4B).

At that time, the GH cell 12 is in a state in which the GH cell 12 doesnot absorb light (incidentally, light is slightly absorbed by the GHcell 12 due to thermal fluctuation, or surface reflection, or the like).Thus, the intensity distribution of light transmitted by the polarizingplate 11 is substantially the same as that of light having passedthrough the opening 22.

Thereafter, the polarizing plate 11 is put into a state in which thepolarizing plate 11 completely covers the opening 22 (FIG. 4C). In acase where the object becomes brighter still more, the light controloperation is performed by raising the voltage supplied to the GH cell 12and absorbing light through the use of the GH cell 12.

Conversely, in a case where the object becomes dark, the effect ofabsorbing light through the use of the GH cell 12 is reduced bydecreasing the voltage applied to the GH cell 12 or applying no voltagethereto. In a case where the object becomes darker, the iris blade 18 isdownwardly moved, and the iris blade 19 by driving the motor (notshown). Thus, the polarizing plate 11 is moved outwardly from theeffective optical path 20 (FIG. 4A).

In the above-mentioned way, the polarizing plate 11 (whose transmittanceranges, for example, 40% to 50%) can be moved out of the effectiveoptical path of light. Thus, light is not absorbed by the polarizingplate 11. Therefore, the maximum light transmittance of the lightcontrol device can be increased by a factor of, for instance, two ormore. Concretely, the maximum light transmittance of this light controldevice is about twice that of the conventional light control deviceconstituted by the polarizing plate and the GH cell, which are fixedlyinstalled therein. Incidentally, both the control devices are equal inthe minimum light transmittance to each other.

Additionally, the polarizing plate 11 is taken in and out by using themechanical iris, which is put in practical use in digital camera and thelike. Thus, the light control device can easily be realized. Also,because the GH cell 12 is used, in addition to the light controloperation performed by the polarizing plate 11, a light controloperation is conducted by the absorption of light by the GH cell 12itself.

Thus, this light control device is enabled to enhance a light-darkcontrast ratio and maintain a substantially uniform light quantitydistribution.

Hereinafter, preferred examples of the invention are described withreference to the accompanying drawings.

EXAMPLE 1

First, an example of the light control device using the guest-hostliquid crystal (GH) cell is described.

The light control device according to the invention is disposed at afront position of the optical lowpass filter in the optical path of theimaging system of the imaging device, and has the polarizing plate 11and the GH cell 12, which are arranged in this order.

Incidentally, as shown in FIG. 1A, the optical lowpass filter 55 bcomprises a birefringent plate 32 a whose light ray separation directionfrom the light incidence side is a horizontal direction, a quarter-wavephase difference plate (whose thickness is, for example, about 0.5 mm)24, and a birefringent plate 32 b whose light ray separation directionis a perpendicular direction.

The GH cell 12 is configured so that a mixture of negative liquidcrystal molecules (a host material) and a positive or negative dichroicdye molecules (a guest material) is sealed between two glass substrates(both are not shown), on each of which a transparent electrode and analignment film are formed.

The MLC-6608 manufactured by Merck Ltd., which is a negative liquidcrystal whose dielectric constant anisotropy) is positive, was used asan example of the liquid crystal molecules. As the positive dichroic dyemolecules 4, D5, which is positive dichroic dye molecules absorbinglight in the molecular long axis direction and manufactured by BDH Ltd.,was used as an example thereof. The light absorption axis of thepolarizing plate 11 was set to be perpendicular to that at the time ofapplying a voltage to the GH cell 12.

Further, a liquid crystal orientation process was performed by using anordinary rubbing method as illustrated in FIG. 5. According to therubbing method, the alignment process performed on the molecules towardthe direction of rotation of a roller 8 can be performed by installing asubstrate 6, which is provided with an alignment film, on a stage 7 of arubbing device and causing the roller 8, which has a rubbing cloth 10,to pass therethrough.

Furthermore, as shown in FIG. 1A, a polarization axis 14 of thepolarizing plate 11 to be inserted to the optical path was disposed bybeing inclined 45 degrees, that is, in such a way as to form an angle of45 degrees with each of (horizontal and vertical) light ray separationdirections of the birefringent plates 32 a and 32 b constituting anoptical lowpass filter 55 b.

Further, as shown in FIG. 6, the device was configured so that thedirection of the liquid crystal orientation of the GH cell 12 and thedirection of the polarization axis 14 of the polarizing plate 11 wereperpendicular to each other.

The light control device 23 constituted by this polarizing plate 11 andthe GH cell 12 is disposed between a front lens group 15 and a rear lensgroup 16, each of which is constituted by a plurality of lenses like azoom lens, as illustrated in, for example, FIG. 2. Light having passedthrough the front lens group 15 is linearly polarized through thepolarizing plate 11, and then incident upon the GH cell 12. Light havingpassed through the GH cell 12 is converged by the rear lens group 16,and projected on an imaging surface 17 as an image.

Incidentally, in the case of a light ray separation pattern 60 accordingto Example 1, a light ray, which forms an image at one point, is splitinto those corresponding to square four points, as shown in FIG. 1A.Further, a splitting axis D at that time can be expressed as being about0.0059×T where T designates the thickness of each of two birefringentplates 32 a and 32 b.

The light control device according to Example 1 is configured so thatthe direction of the polarization axis 14 of the polarizing plate 11 andthe direction of the liquid crystal orientation 25 of the GH cell 12form an angle of 45 degrees with the light ray separation directions ofthe birefringent plates 32 a and 32 b constituting the optical lowpassfilter 55 b. Thus, for example, even in a case where the light controldevice, the optical lowpass filter 55 b, and the CCD element accordingto Example 1 are disposed in a casing, and where even when geometricalpatterns (of, for example, objects, which have high spatial frequencies,to be imaged, such as striped clothes, and tiled walls of buildings),which are finer than the periodic arrangement of the CCDs, are imaged,no deviation of the intensity of the separated light occurs. The effectof blurring can sufficiently be obtained. The generation of false colorsignals and moirés can effectively be prevented. A picked-up image,which has faithfully reproduced an imaged object and is a more naturalimage, can be obtained.

Further, the GH cell 12 is constituted by using the negative hostmaterial thereby to enhance the light transmittance (especially, whenthe cell is transparent) and to realize a more compact light controldevice enabled to use the GH cell 12 by fixing the position thereof inthe imaging optical system without change. In this case, the polarizingplate is disposed in the optical path of light that is incident upon theliquid crystal element. Consequently, the ratio of absorbance onapplication of a voltage to absorbance on application of no voltage(that is, the optical density ratio) is further enhanced. The contrastratio of the light control device is further increased. A light controloperation can normally be performed at all places from light places todark places.

Incidentally, the polarizing plate 11 constituting this light controldevice can be taken in and out of the effective optical path 20 of lightthat is incident upon the GH cell 12, similarly to the inventiondescribed in the earlier application filed by the applicant of thepresent application.

Concretely, as shown in FIG. 2, the polarizing plate 11 can be taken outof the effective optical path 20 of light by being moved to a positionindicated by imaginary lines. The mechanical iris shown in FIGS. 3 and 4may be used as the means for taking this polarizing plate 11 therein andout therefrom.

EXAMPLE 2

The differences between Example 2 and Example 1 reside in that themembers constituting the optical lowpass filter of Example 2 differ fromthose of Example 1, and that the direction of the polarization axis ofthe polarizing plate and the direction of the liquid crystal orientationof the GH cell are changed so as to correspond to this optical lowpassfilter.

That is, in Example 2, as shown in FIG. 1B, the optical lowpass filter55 b comprises a birefringent plate 32 b whose light ray separationdirection is a perpendicular direction, and two birefringent plates 32 cand 32 d, whose light ray separation directions are apart therefrom 45degrees.

Furthermore, the polarizing plate 11 was disposed so that the directionof the polarization axis 14 was a horizontal direction. That is, thedevice was configured so that the direction of the polarization axis 14of the polarizing plate 11 differed from the light ray separationdirections of the birefringent plates 32 b, 32 c, and 32 d constitutingthe optical lowpass filter 55 b.

Further, as shown in FIG. 7, the device was configured so that thedirection of the liquid crystal orientation 25 of the GH cell 12 and thedirection of the polarization axis 14 of the polarizing plate 11 wereperpendicular to each other.

In the case of a light ray separation pattern 60 according to Example 2,a light ray, which forms an image at one point, is split into thosecorresponding to square four points, as shown in FIG. 1(B). Further, asplitting axis D at that time can be expressed as being about 0.0059×Twhere the thickness of the birefringent plate 32 b, whose light rayseparation direction is a perpendicular direction, is designated by T,and the thickness of each of the birefringent plates 32 c and 32 d,whose light ray separation directions are apart therefrom 45 degrees, isset to be $\frac{T}{\sqrt{2}}.$

The light control device according to Example 2 is configured so thatthe direction of the polarization axis 14 of the polarizing plate 11 andthe direction of the liquid crystal orientation 25 of the GH cell 12differ from the light ray separation directions of the birefringentplates 32 b, 32 c and 32 d constituting the optical lowpass filter 55 b.Thus, for example, even in a case where the light control device, theoptical lowpass filter 55 b, and the CCD element according to Example 2are disposed in a casing, and where even when geometrical patterns (of,for example, objects, which have high spatial frequencies, to be imaged,such as striped clothes, and tiled walls of buildings), which are finerthan the periodic arrangement of the CCDs, are imaged, the generation offalse color signals and moirés can more effectively be prevented, and apicked-up image, which has faithfully reproduced an imaged object and isa further more natural image, can be obtained, similarly to Example 1.

EXAMPLE 3

The differences between Example 3 and Example 1 reside in that themembers constituting the optical lowpass filter of Example 3 differ fromthose of Example 1, and that the direction of the polarization axis ofthe polarizing plate and the direction of the liquid crystal orientationof the GH cell are changed so as to correspond to this optical lowpassfilter.

That is, in Example 3, as shown in FIG. 1C, the optical lowpass filter55 b comprises a birefringent plate 32 a whose light ray separationdirection is a horizontal direction, and two birefringent plates 32 eand 32 c, whose light ray separation directions are apart therefrom 45degrees.

Further, the polarizing plate 11 was disposed so that the direction ofthe polarization axis 14 was a perpendicular direction. That is, thedevice was configured so that the direction of the polarization axis 14of the polarizing plate 11 differed from the light ray separationdirections of the birefringent plates 32 a, 32 e, and 32 c constitutingthe optical lowpass filter 55 b.

Further, the device was configured so that the direction of the liquidcrystal orientation 25 of the GH cell 12 and the direction of thepolarization axis 14 of the polarizing plate 11 were perpendicular toeach other.

In the case of a light ray separation pattern 60 according to Example 3,a light ray, which forms an image at one point, is split into thosecorresponding to square four points, as shown in FIG. 1C. Further, asplitting axis D at that time can be expressed as being about 0.0059×Twhere the thickness of the birefringent plate 32 a, whose light rayseparation direction is a horizontal direction, is designated by T, andthe thickness of each of the birefringent plates 32 e and 32 c, whoselight ray separation directions are apart therefrom 45 degrees, is setto be $\frac{T}{\sqrt{2}}.$

The light control device according to Example 3 is configured so thatthe direction of the polarization axis 14 of the polarizing plate 11 andthe direction of the liquid crystal orientation 25 of the GH cell 12differ from the light ray separation directions of the birefringentplates 32 a, 32 e and 32 c constituting the optical lowpass filter 55 b.Thus, for example, even in a case where the light control device, theoptical lowpass filter 55 b, and the CCD element according to Example 3are disposed in a casing, and where even when geometrical patterns (of,for example, objects, which have high spatial frequencies, to be imaged,such as striped clothes, and tiled walls of buildings), which are finerthan the periodic arrangement of the CCDs, are imaged, the generation offalse color signals and moirés can more effectively be prevented, and apicked-up image, which has faithfully reproduced an imaged object and isa further more natural image, can be obtained, similarly to Example 1.

EXAMPLE 4

The differences between Example 4 and Example 1 reside in that themembers constituting the optical lowpass filter of Example 4 differ fromthose of Example 1, and that the direction of the polarization axis ofthe polarizing plate and the direction of the liquid crystal orientationof the GH cell are changed so as to correspond to this optical lowpassfilter.

That is, in Example 4, as shown in FIG. 1D, the optical lowpass filter55 b comprises a birefringent plate 32 a whose light ray separationdirection is a horizontal direction, and a birefringent plate 32 d,whose light ray separation direction is apart therefrom 45 degrees.

Further, the polarizing plate 11 was disposed so that the direction ofthe polarization axis 14 was a perpendicular direction. That is, thedevice was configured so that the direction of the polarization axis 14of the polarizing plate 11 differed from the light ray separationdirections of the birefringent plates 32 a and 32 d constituting theoptical lowpass filter 55 b.

Further, the device was configured so that the direction of the liquidcrystal orientation 25 of the GH cell 12 and the direction of thepolarization axis 14 of the polarizing plate 11 were perpendicular toeach other.

In the case of a light ray separation pattern 60 according to Example 4,a light ray, which forms an image at one point, is split into thosecorresponding to rhombic four points, as shown in FIG. 1D. Further, asplitting axis D at that time can be expressed as being about 0.0059×Twhere the thickness of the birefringent plate 32 a, whose light rayseparation direction is a horizontal direction, is designated by T, andthe thickness of the birefringent plate 32 d, whose light ray separationdirection is apart therefrom 45 degrees, is set to be$\frac{T}{\sqrt{2}}.$

The light control device according to Example 4 is configured so thatthe direction of the polarization axis 14 of the polarizing plate 11 andthe direction of the liquid crystal orientation 25 of the GH cell 12differ from the light ray separation directions of the birefringentplates 32 a and 32 d constituting the optical lowpass filter 55 b. Thus,for example, even in a case where the light control device, the opticallowpass filter 55 b, and the CCD element according to Example 4 aredisposed in a casing, and where even when geometrical patterns (of, forexample, objects, which have high spatial frequencies, to be imaged,such as striped clothes, and tiled walls of buildings), which are finerthan the periodic arrangement of the CCDs, are imaged, the generation offalse color signals and moirés can more effectively be prevented, and apicked-up image, which has faithfully reproduced an imaged object and isa further more natural image, can be obtained, similarly to Example 1.

EXAMPLE 5

FIG. 8 shows an example of incorporating the light control deviceaccording to Example 1 into a CCD (Charge Coupled Device) camera.

That is, in the CCD camera 50, along the optical axis indicated by adot-dash line, a first group lens 51 and a second group lens (forzooming) 52, which correspond to the front lens group 15, a third grouplens 53 and a fourth group lens 54 (for focusing), which correspond tothe rear lens group 16, and a CCD package 55 are disposed at appropriateintervals in this order. In the CCD package 55, an infrared ray cut-offfilter 55 a, an optical lowpass filter 55 b, and a CCD imaging element55 c are accommodated.

The light control device according to the invention, which comprises thepolarizing plate 11 and the GH cell 12, are disposed between the secondgroup lens 52 and the third group lens 53 in such a way as to be closerto the third group lens 53. Incidentally, the fourth group lens 54 forfocusing is disposed in such a manner as to be movable by using a linearmotor along the optical path between the third group lens 53 and the CCDpackage 55. Also, the second group lens 54 for zooming is disposed insuch a way as to be movable between the first group lens 51 and thelight control device 23.

FIG. 9 illustrates an algorithm of a control sequence for controllingthe light transmittance by using the light control device 23 in thiscamera system.

According to this Example, the light control device 23 based on theinvention is provided between the second group lens 52 and the thirdgroup lens 53. Thus, as above-mentioned, the quantity of light can becontrolled by applying a voltage. The system can be miniaturized andreduced in size to substantially the size of the effective range of theoptical path. Consequently, the miniaturization of a CCD camera can beachieved. Further, the quantity of light can appropriately be controlledaccording to the magnitude of the voltage applied to patternedelectrodes. Thus, a diffraction phenomenon, which would occur in theconventional device, can be prevented from occurring. A sufficientquantity of light is incident upon the imaging element. Consequently,the blurring of an image can be prevented.

Although Examples according to the invention have been described in theforegoing description, the above-mentioned Examples may be modified invarious manners according to the technical ideas according to theinvention.

Needless to say, for example, the sample structure, the used materials,the drive method for the GH cell 12, the configuration of the lightcontrol device, and so fourth can be appropriately selected withoutdeparting the spirit and scope of the invention.

Furthermore, although examples using the most ordinary rubbing method asthe method for establishing the liquid crystal alignment of the GH cell12 have been described in the description of Examples, the invention canbe applied to cases of using liquid crystal alignment method that employan oblique evaporation film, a light alignment film or structures or thelike formed by polarized radiation.

Additionally, although the example of using Pulse Height Modulation(PHM) as the drive method for the GH cell 12 has been described in theforegoing description, the invention can be applied to a case of drivingthe GH cell according to Pulse Width Modulation (PWM).

Furthermore, in addition to the above-mentioned GH cells, a GH cell of atwo-layer structure may be used as the GH cell 12. The position of thepolarizing plate 11 with respect to the GH cell 12 may be set at anoptimal place determined according to conditions for setting an imaginglens.

Further, although the example, in which absorption of light is performedby the GH cell 12 after a light control operation is first performed bytaking the polarizing plate 11 therein or out therefrom, has beendescribed, conversely, the operation of controlling light by absorbinglight through the use of the GH cell 12 may be first performed. In thiscase, it is preferable that the light control operation by taking thepolarizing plate 11 in and out is performed after the transmittance ofthe GH cell 12 is reduced to a predetermined value.

Furthermore, although the mechanical iris is used as the means fortaking the polarizing plate 11 into and out of the effective opticalpath 20 in Example, this means is not limited thereto. The polarizingplate 11 may be taken in and out by a film, to which the polarizationplate 11 is attached, may be directly provided in the drive motor.

The number of the iris blades 18 and 19 is not limited to 2. A largernumber of iris blades may be used. Conversely, only one iris blade maybe used. Additionally, although the iris blades 18, 19 are overlapped bybeing moved in the up-down direction, the iris blades may be moved inother directions. The iris may be closed from the periphery thereof tothe center thereof.

Further, although the polarizing plate 11 is attached to the iris blade18, the polarization plate 11 may be attached to the iris blade 19.

Furthermore, the light control device according to the invention may beused by being combined with other known filter materials (for example,an organic electrochromic material, a liquid crystal, anelectroluminescent material, and the like).

Additionally, the light control device according to the invention can beapplied to various optical systems, for instance, means for controllingthe quantity of light in an electrophotographic copier and opticalcommunication equipment and the like in addition to the opticaldiaphragm of the imaging device, such as the CCD camera. Moreover, thelight control device according to the invention can be applied tovarious kinds of image display devices for displaying characters andimages, in addition to the optical diaphragm and the filter.

Further, in addition to the CCDs used in Example, CMOS image sensors maybe applied as pickup devices used in the device according to theinvention.

INDUSTRIAL APPLICABILITY

According to the invention, the device is configured so that thedirection of a polarization axis of the polarizing plate and thedirection of a liquid crystal orientation differ from light rayseparation direction of an ordinary ray and an extraordinary rayseparated by the optical lowpass filter. Thus, for instance, whengeometrical patterns (of, for example, objects, which have high spatialfrequencies, to be imaged, such as striped clothes, and tiled walls ofbuildings striped clothes, and tiled walls of buildings), which arefiner than the periodic arrangement of the CCDs, are imaged, nodeviation of the intensity of the separated light occurs. The effect ofblurring can sufficiently be obtained. Occurrences of false colorsignals and moirés can effectively be prevented. A picked-up image,which has faithfully reproduced an imaged object and is a more naturalimage, can be obtained. Even when the directions of the polarizationaxis and the liquid crystal orientation are parallel to the light rayseparation direction, a deviation of the intensity of the separatedlight occurs, so that the number of the separated light rays decreases,and that the effect of blurring is not obtained.

Consequently, the invention can enhance the optical functions of thelight control device and the imaging device and is extremely effectivein enhancing the performance, the quality of an image, and thereliability of the device.

1. A light control device, provided at a front position of an opticallowpass filter comprising a plurality of birefringent plates,sequentially disposed in an optical path of an imaging system of animaging device, said light control device comprising: a liquid crystalelement and a polarizing plate; wherein the direction of a polarizationaxis of said polarizing plate and that of liquid crystal alignment ofsaid liquid crystal element differ from light ray separation directionof an ordinary ray and an extraordinary ray, which are separated by anoptical lowpass filter; and light ray separation directions of saidplural birefringent plates and the direction of the polarization axis ofsaid polarizing plate wholly differ from one another.
 2. An imagingdevice, in which a light control device having a polarizing plate and aliquid crystal element is disposed at a front position of an opticallowpass filter comprising a plurality of birefringent plates,sequentially disposed in an optical path of an imaging system; whereinthe direction of a polarization axis of said polarizing plate and thatof a liquid crystal orientation differ from light ray separationdirection of an ordinary ray and an extraordinary ray separated by saidoptical lowpass filter; and light ray separation directions of saidplural birefringent plates and the direction of the polarization axis ofsaid polarizing plate wholly differ from one another.
 3. (canceled) 4.The light control device according to claim 1, or the imaging deviceaccording to claim 2; wherein the direction of the polarization axis ofsaid polarizing plate and that of liquid crystal orientation of saidliquid crystal element forms an angle of 45 degrees with at least thelight ray separation direction of a birefringent plate that is presentclosest to a light incidence side among members constituting saidoptical lowpass filter.
 5. The light control device according to claim1, or the imaging device according to claim 2; wherein the direction ofthe polarization axis of said polarizing plate and that of the liquidcrystal orientation of said liquid crystal element are perpendicular toeach other.
 6. The light control device according to claim 1, or theimaging device according to claim 2; wherein said liquid crystal elementis a guest-host liquid crystal element that employs negative liquidcrystal molecules as a host material, and that also employs diachronicdye molecules as a guest material.
 7. The light control device accordingto claim 1, or the imaging device according to claim 2; wherein saidimaging device is a CCD (Charge Coupled Device) camera.
 8. The lightcontrol device according to claim 1, or the imaging device according toclaim 2; wherein said light control device, said optical lowpass filter,and said CCD (Charge Coupled Device) element are disposed in a casing.